Two-Step Conversion of Biomass-Derived Glucose with High

Article pubs.acs.org/IECR

Two-Step Conversion of Biomass-Derived Glucose with High Concentration over Cu−Cr Catalysts Zihui Xiao,† Shaohua Jin,† Guangyan Sha,† Christopher T. Williams,‡ and Changhai Liang*,† †

Laboratory of Advanced Materials and Catalytic Engineering, Dalian University of Technology, Panjin, Liaoning 124221, China Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States

‡

S Supporting Information *

ABSTRACT: The conversion of highly concentrated glucose was conducted over a Cu−Cr catalyst with a base in two steps for the first time. Reaction parameters such as reaction time, temperature, and H2 pressure were optimized in each step. On the basis of these results, the corresponding reaction route was proposed. At the low-temperature step, and without a base, glucose was hydrogenated into sorbitol or isomerized and hydrogenated into mannitol. While in the presence of a base, the direct decomposition of glucose was observed because of base-catalyzed retro-aldol condensation. At the high-temperature step, it was found that the addition of a base greatly restrained the formation of oligosaccharides. Compared with CaCO3, Ba(OH)2, KOH, and NaOH, Ca(OH)2 exhibited the best promotion, indicating that the conversion of glucose was relative to not only the concentration of OH− but also the metal ionic radius and electric charge. The addition of a base had no obvious effect on the stability of Cu−Cr catalyst. In the recycling, the catalyst exhibited reasonable recyclability because of the partial deactivation originating from the migration of Cr onto Cu sites and the coverage of carbon species, as shown by X-ray photoelectron spectroscopy measurements. K and 6.9 MPa H2.7 To avoid the condensation of glucose, the hydrogenolysis of sorbitol originating from the hydrogenation of glucose was explored in many reports.8−10 However, the hydrogenolysis was performed generally in the alkaline solution by adding a base as a promoter. In alkaline solutions, the hydrogenolysis of cellulose and its derived polyols has been extensively reported.11−15 The selectivity of ethylene glycol and propylene glycol were relatively higher on Ru/C catalyst in aqueous basic solution.11 The phenomenon was attributed to the fact that hydroxide ions could not only readily break the hydrogen bonds in cellulose to finally form sugars but also catalyze isomerization and retroaldol condensation of carbohydrates leading to C−C bond cleavage and degradation. Clark et al. reported that sorbitol could be hydrogenolyzed to ethylene glycol, propylene glycol, and glycerol in yields of 16%, 17% and 40%, respectively, at 488 K and 14.0 MPa H2 in water on kieselguhr-supported Ni catalysts with Ca(OH)2 as a promoter.12 Meanwhile, it was found that in a basic medium the rate of sorbitol hydrogenolysis over Ru catalyst was about twice as high as that obtained in a neutral medium.13 For the selective hydrogenolysis of biomassderived xylitol, in the presence of the solid bases, the selectivity of ethylene glycol and propylene glycol was significantly increased, and the activities increased almost linearly with increasing pH values.14 Addition of a base also enhanced the performance of carbon-supported Pt and Ru in the batchwise hydrogenolysis of glycerol in aqueous solution.15 In these reports, addition of a base greatly promoted the retro-aldol

1. INTRODUCTION With the decline of fossil feedstock and increasing use of them, combined with political and environmental concerns, it is imperative to develop efficient processes for the conversion of renewable biomass resources into fuels and chemicals to avoid intensification of global warming and to diversify energy sources.1 Cellulose, as an important sustainable biomass resource, has been reported to be transformed into polyols over various noble metal2 and transition-metal catalysts.3,4 In the hydrogenolysis of cellulose, large amounts of water-soluble saccharides, including polysaccharides and monosaccharides (formed by subsequent hydrolysis of polysaccharides) are easily recondensed into polymers with high molecular weight which produce coke-like precipitates at high reaction temperatures. Developing a catalyst with excellent activity for the hydrogenolysis of saccharides to avoid undesired condensation will therefore be required for the effective hydrogenolysis of cellulose. As a model substrate for saccharides, glucose is a good choice for evaluating catalyst performance on hydrogenolysis of cellulose. Glucose (C6H12O6) is the most abundant hexose in nature. Significant previous efforts have been made to accomplish its hydrogenolysis.5−7 In the first successful study, the hydrogenolysis of glucose (50%) was investigated over kieselguhrsupported Ni catalyst with CaCO3 as a promoter, with a maximum obtained conversion of 71.5%.5 Over Os on activated carbon catalyst, glucose hydrogenolysis with the concentration of 26% was converted into the two products of ethylene glycol and propylene glycol with a combined yield as high as 46.6% in the presence of Ba(OH)2 at 483 K and 5.0 MPa H2.6 Using a homogeneous catalyst of ruthenium acetylacetonate, 28% concentrated glucose could convert into ethylene glycol, propylene glycol, and glycerol with total yield of 61% at 523 © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8735

March 23, 2014 May 4, 2014 May 6, 2014 May 6, 2014 dx.doi.org/10.1021/ie5012189 | Ind. Eng. Chem. Res. 2014, 53, 8735−8743

Industrial & Engineering Chemistry Research

Article

The liquid products were analyzed with a Waters 1525 highperformance liquid chromatography (HPLC) instrument equipped with a refractive index detector (Capcell PAK NH2 UG80S5; column, 4.6 × 250 mm2) and a Bruker 450-GC gas chromatography (GC) instrument equipped with a flame ionization detector (FFAP GC; column, 30 m × 0.32 mm × 0.5 μm). The gas products were analyzed with a 7890F gas chromatograph equipped with thermal conductivity detector (TDX-01; column, 4 mm × 2 m). The conversion of glucose was determined by the equation conversion (%) = [(weight of glucose put into the reactor − weight of residual glucose)/ weight of glucose put into the reactor] × 100. The yield of liquid product was calculated from the equation yield (%) = (weight of target product)/(weight of glucose put into the reactor) × 100.

condensation based on the C−C cleavage mechanism of the base catalyzed reaction.13 This promotion generally contributed to the high concentration of OH− in the solution. However, the underlying reasons are not clear, and the role of metal ions (e.g., Ca2+, Ba2+, K+, Na+), is not comprehensively investigated. In previous work, we found that Cu−Cr catalysts exhibited an excellent catalytic performance and good resistance against coking for the hydrogenolysis of highly concentrated cellulose.16 Moreover, the addition of base greatly increased the yield of ethylene glycol. Therefore, in the present report, the conversion of highly concentrated glucose over Cu−Cr catalysts was examined to explore the mechanism of coking and role of base in the reaction. Compared with cellulose, highly concentrated glucose was employed because of the relatively low molecular weight compared to that of the polysaccharides presented in the conversion of cellulose. Two-step conversion of glucose was accomplished and the formation of coke-like precipitates was efficiently avoided. Reaction parameters such as reaction time, temperature, and H2 pressure were optimized at each step. Finally, the role of metal ions as well as the stability and recyclability of the catalysts was investigated.

3. RESULTS AND DISCUSSION In the case of the Cu−Cr catalysts with different molar ratios, we found that the CuCr(4) catalyst gave the best performance for the hydrogenolysis of glycerol and cellulose. Thus, it was evaluated for the catalytic conversion of glucose. The structural properties of the CuCr(4) catalyst were investigated comprehensively in our previous works.18,19 The XRD pattern showed that the main crystalline phase of CuO with low diffraction intensity of CuCr2O4 was detected. For further understanding of the catalyst structure, a structure of CuO supported CuCr2O4 was identified in the HRTEM images, which was also proven by XPS and H 2 temperatureprogrammed reduction measurements. The scanning transmission electron microscopy energy-dispersive X-ray mapping exhibited that the dispersion of Cu and Cr species was homogeneous. 3.1. Conversion of Glucose in Two Steps. For comparison, the conversion of highly concentrated glucose was performed by one step in the presence or absence of base, and the corresponding reaction results are listed in Table S1 of the Supporting Information. First, one-step conversion of highly concentrated glucose was conducted at 493 K and 6 MPa of H2. Irrespective of the presence of base, a large amount of coke-like precipitates formed (entries 1 and 2). When the temperature was reduced to 413 K, it was found that glucose could be hydrogenated into sorbitol (SOR) and mannitol (MAN) (C6 products) in the absence of base (entry 3). While in the presence of Ca(OH)2 (entries 4−6), the enhancement of base-catalyzed retro-aldol condensation led to the formation of polyols with low molecular weight (C2−4 products), such as 1,2-propanediol (1,2-PD), ethylene glycol (EG), glycerol (GLY), and erythritol (ERY). Within 5 h (entry 6), a combined yield of C2−4 products as high as 49.2% was obtained. When the reaction time was increased, an increase in the yields of C6 and C2−4 products was observed, accompanied by a decrease in the yield of oligosaccharides, indicating that the formed oligosaccharides could be also converted into C6 and C2−4 products. It appears that the condensation rate of glucose was higher than the decomposition rate of glucose at the beginning because a large amount of oligosaccharides was obtained within a short reaction time. Clearly, less than 100% total yield was obtained because of the formation of polysaccharides by further condensation of oligosaccharides, which could not be determined by the GC or HPLC under the present conditions. To improve the yield of C2−4 products, high temperature was required for the conversion of obtained C6 products and saccharides. Thus, a two-step transformation process of glucose

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Cu−Cr catalysts were synthesized by a nonalkoxide sol−gel route.17−19 In a typical synthesis, 11.7 g of Cu(NO3)2·3H2O and 4.8 g of Cr(NO3)3· 9H2O at a desired molar ratio (Cu/Cr = 4) were dissolved in 24 mL of ethanol at 333 K to give a clear dark blue solution. After 9 mL of 1,2-propylene oxide were added to the solution, a dark green transparent gel formed within a few minutes. The obtained wet gel was aged under air atmosphere and then dried at 348 K for 15 h, and the resulting xerogel was calcined at 773 K for 120 min. The prepared catalysts were designated as CuCr(4). 2.2. Catalyst Characterization. X-ray diffraction (XRD) patterns of the samples were measured in a D/MAX-2400 diffractometer with a Cu Kα monochromatized radiation source (λ = 1.5418 Å) operated at 40 kV and 100 mA. X-ray photoelectron spectroscopic (XPS) data were obtained using an ESCALAB250 Surface Science instrument. A monochromatic Al Kα (1486.6 eV) X-ray source was used as the incident radiation. The base pressure in the measurement chamber was 2 × 10−10 mbar. The analyzer slit was set to 0.4 mm, and a pass energy of 200 eV was chosen, resulting in an overall energy resolution better than 0.5 eV. Charging effects were compensated by the usage of a flood gun. The binding energies were calibrated based on the C 1s peak at 284.5 eV as a reference. The XPS peaks were analyzed using a Shirley-type background and a nonlinear least-squares fitting of the experimental data based on a mixed Gaussian−Lorentzian peak shape. 2.3. Catalytic Reaction. The conversion of glucose was performed in a 50 mL stainless steel autoclave with mechanical stirrer and an electric temperature controller, operated under H2 pressure. Prior to the reaction, the prepared catalysts were reduced by 10% H2 in Ar at 573 K for 2 h. The autoclave was charged with 27 g of aqueous solution of 30 wt % glucose and 0.4 g catalyst. The reactor was sealed and pressurized to the required hydrogen pressure and then heated to the desired temperature for the first stage reaction. After the reaction, the autoclave was cooled to room temperature, pressurized, and heated again to desired condition for the second stage reaction. A discontinuous process was taken to supplement hydrogen. 8736

dx.doi.org/10.1021/ie5012189 | Ind. Eng. Chem. Res. 2014, 53, 8735−8743


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Table 1. Two-Step Conversion of Glucose or Sorbitol over CuCr(4) Catalyst with Ca(OH)2 reaction conditions entry

substrates (30 wt %)

1 2 3 4 5 6

glucose glucose glucose glucose glucosee sorbitol

first 383 398 413 413 413 413

K, K, K, K, K, K,

6 6 6 6 6 6

MPa, 1 h MPa, 1 h MPa, 1 h MPa, 2 h Mpa, 2 h MPa, 2 h

yield (%) second

493 493 493 493 493 493

K, K, K, K, K, K,

6 6 6 6 6 6

MPa, MPa, MPa, MPa, MPa, MPa,

5 5 5 5 5 5

h h h h h h

conv. (%)

1,2-PD

C2−4a

C6b

oligos.c

othersd

>99.9 >99.9 >99.9 >99.9 >99.9 56.8

34.4 43.7 50.2 52.8 5.4 43.5

48.6 63.1 71.3 76.9 10.1 54.8

28.0 22.3 18.3 20.9 38.9f 1.0

10.0 5.6 1.1 0.2 30.2 −

4.6 3.4 2.9 1.1 9.1 0.6

a

C2−4: EG, 1,2-PD, GLY, and ERY. bC6: SOR and MAN for the conversion of glucose, MAN for the conversion of sorbitol. cWater-soluble oligosaccharides. dOthers: liquid (methanol, ethanol, propanol, furfuran, and some unknown), gas (CO2, CH4, CO). eWithout Ca(OH)2; f6.7% of SOR and MAN total yield; 14.3% of ISO yield; 17.9% of 1,4-SB yield.

Scheme 1. Conversion of Glucose over Cu−Cr Catalyst with or without Base

enhanced by elevating reaction temperature in the first step, the conversion rate was also improved. Thus, in the range of 383−413 K, the yield of C2−4 was almost linearly increased with the reaction temperature (entries 1−3). The reason was that higher temperature was beneficial to the further conversion of formed saccharides and C6 products. However, out of this temperature range, serious coking was observed. At low temperature, the formed large amount of saccharides was quickly polymerized into coke-like precipitates in the second step; otherwise, this occurs in the first step at higher temperature. For comparison, the conversion of sorbitol under conditions that were the same as those in the presence of Ca(OH)2 was also conducted (entry 6). Lower conversion reflects that sorbitol has a thermal stability higher than that of glucose.8 This result indicates that the C2−4 products with high yield are mainly obtained by the direct conversion of glucose and/or saccharides rather than sugar alcohol, although the relative intermediates were not identified. On the basis of the above suggestion, the reaction route of glucose is proposed as shown in Scheme 1. Glucose can be hydrogenated to sorbitol or isomerize to fructose via keto−enol

was designed wherein the low-temperature reaction step was employed for restraining a high degree of glucose condensation and even formation of coke. Table 1 shows the reaction results from the two-step conversion of glucose. As expected, the C2− 4 products summing to a combined yield of 76.9% (entry 4) were obtained in the presence of base with two steps. The maximum yield of 1,2-PD reached 52.8%. To the best of our knowledge, this is the highest yet achieved in the hydrogenolysis of highly concentrated glucose as listed in Table S2 of the Supporting Information. However, without addition of Ca(OH)2, 1,4-sorbitan (1,4-SB) and isosorbide (ISO) with 17.9% and 14.3% yield, respectively, were obtained from C−O bond cleavage (entry 5). Meanwhile, only a small amount of C−C bond cleavage products were detected, indicating that the addition of base significantly changed the reaction route. Clearly, it was known that the addition of base greatly restrained the formation of oligosaccharides, e.g., decreased from 30.2% to 0.2%. As stated above, the condensation rate of saccharides was higher than the conversion rate of saccharides. In a certain temperature range, although the condensation rate was 8737



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Figure 1. Effect of reaction time (a) and H2 pressure (b) on the conversion of glucose in the first step (Reaction conditions: 27 g 30 wt % glucose aqueous solution; CuCr(4), 0.4 g; Ca(OH)2, 0.08 g. First step: 413 K, 6 MPa, 2 h. Second step: 493 K, 6 MPa, 5 h; 900 rpm; higher than 99.9% of glucose conversion).

Figure 2. Effect of reaction time (a), temperature (b), and H2 pressure (c) on the conversion of glucose in the second step. (Reaction conditions: 27 g 30 wt % glucose aqueous solution; CuCr(4), 0.4 g; Ca(OH)2, 0.08 g. First step: 413 K, 6 MPa, 2 h. Second step: 493 K, 6 MPa, 5 h, 900 rpm; higher than 99.9% of glucose conversion).

tautomerization and then be hydrogenated to mannitol.20 Further dehydration of sorbitol forms isosorbide via 1,4sorbitan. This process is restrained in the alkaline solution. In the presence of base, the base-promoted retro-aldol condensation of glucose leads to the formation of C2−4 products. Pioneering researchers have reported that dehydrogenation is a necessary step in the hydrogenolysis of sugar alcohols.21 The corresponding carbonyl intermediates could decompose into two C3 products or C2 and C4 products (depending on the position of dehydrogenation) by the retro-aldol condensation in the base solution.13 Obviously, the C2−4 products could also be obtained by the conversion of sorbitol and mannitol. In

addition, the formed water-soluble oligosaccharides could convert into C6 and C2−4 products as well, which could further convert into other chemicals, such as methanol, ethanol, propanol, furan compounds, CO2, CH4, CO, etc. In our previous work we compared the present CuCr(4) catalyst to the other typical hydrogenolysis catalysts. The Cu− Cr catalyst exhibited a good resistance against coking in the hydrogenolysis of highly concentrated cellulose.16 Similarly, in the present case, 2%Ni-30%W/AC, 3%Pt-1%Ru/C, Ni−Cr, and Cu−Fe were tested in the hydrogenolysis of highly concentrated glucose with two steps in the presence of Ca(OH)2. Unfortunately, these catalysts gained poor activity 8738



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3.2. Role of Base in the Conversion of Glucose. The influence of pH on the hydrogenolysis of polyols has been reported previously.14 Similarly, the conversion of glucose over the CuCr(4) catalyst was conducted in the presence of various bases, such as Ca(OH)2, CaCO3, Ba(OH)2, NaOH, and KOH (Table 2). Upon addition of CaCO3 with the same amount of

with serious coking at high concentration. These results suggest that the oligosaccharides and/or glucose originating from hydrolysis of cellulose are rapidly converted into other chemicals with low molecular weight over the Cu−Cr catalyst, restraining the condensation and the formation of coke-like precipitates. The outstanding performances might be related to the unique structure as found in the hydrogenolysis of cellulose.16 In the remaining part, glucose was completely converted under given conditions unless otherwise noted. Figure 1 shows the reaction results for highly concentrated glucose in the first step as a function of the reaction time and H2 pressure on CuCr(4) catalyst with Ca(OH)2. As the reaction time increased, C2−4 and C6 products showed the same trend (Figure 1a). Typically, the yield of 1,2-PD increased to the maximum within 2 h and gradually decreased with longer reaction time because of further conversion of it, which was consistent with the previous observation for the hydrogenolysis of glycerol.18 This indicates that the hydrogenation and retroaldol condensation of glucose are parallel under the given conditions. Clearly, the total yield dropped with further increasing reaction time. This was due to the further condensation of oligosaccharides, the cyclization, and partial polycondensation by dehydration of some intermediates at high temperature,22 which could not be determined by the GC or HPLC under the present conditions. The H2 pressure had a positive effect on the yield of C2−4 products as shown in Figure 1b. In addition, the yield of C6 products was increased because of the enhancement of glucose hydrogenation and isomerization at high H2 pressure. The performance of glucose transformation in the second step was examined as a function of reaction time, temperature, and H2 pressure, with the corresponding results shown in Figure 2. With an increase in the reaction time, the yield of C6 increased sharply, reached the maximum value, and then gradually decreased. As displayed in Table S1 (entry 5) of the Supporting Information, the glucose was almost converted within 2 h in the first stage. Thus, the increase in the yield of C6 could be attributed to the conversion of saccharides rather than the hydrogenation or isomerization of glucose. Subsequently, the conversion of C6 products led to an increase in the yield of C2−4 products, as shown in Figure 2a. Nevertheless, the yield of C2−4 products gradually decreased with increasing time to 10 h. This was due to the further conversion of them at high temperature, forming other chemicals as shown in Scheme 1. Figure 2b shows the yields for the glucose conversion as a function of the reaction temperature on the CuCr(4) catalyst. Increasing reaction temperature had a positive effect on the conversion of C6 and saccharides, resulting in an increase in the yield of C2−4 products in the range 473−493 K. When the temperature was further increased, the yield of C2−4 products decreased because of the enhancement of their conversion process. With an extended reaction time or elevated reaction temperature in the second step, the total yield was decreased similar to that shown in Figure 1a, which was also attributed to the cyclization and partial polycondensation.22 The yield of C2−4 and C6 products increased with increasing H2 pressure between 4 to 6 MPa (Figure 2c), which was attributed to the conversion of saccharides. Moreover, the enhancement of further conversion at higher H2 pressure and high temperature led to the decrease of the yield of C2−4 and C6 products.

Table 2. Role of Base in the Conversion of Glucosea yield (%) pH

base

1,2-PD

C2−4

C6

oligos.

others

11.7 9.1 11.7 11.7 11.7

Ca(OH)2 CaCO3 Ba(OH)2 NaOH KOH

52.8 32.6 16.5 17.8 19.1

76.9 46.3 29.5 31.2 29.5

20.9 38.2 44.1 43.5 47.3

0.2 3.2 7.1 7.3 4.9

1.1 3.6 6.8 6.4 3.1

a

Reaction conditions: 27 g 30 wt % glucose aqueous solution; CuCr(4), 0.4 g; 0.03−0.11 g base. First step: 413 K, 6 MPa, 2 h. Second step: 493 K, 6 MPa, 5 h, 900 rpm; higher than 99.9% of glucose conversion.

Ca2+, higher yield of C6 products together with lower yield of C2−4 products was observed because of the weak retro-aldol condensation in the lower alkaline solution, as the pH decreased from 11.7 to 9.1. In alkaline solutions with similar pH, adding Ca(OH)2 gave superior conversion performance compared to that resulting from the addition of Ba(OH)2, NaOH, and KOH, indicating that the type of metal ion also affected the conversion process. As suggested, the role of metal ions might be related to their ionic radius and electric charge. In comparison with Ca2+ (rCa2+ = 0.100 nm), the ionic radius of Ba2+ (rBa2+ = 0.136 nm) or K+ (rK+ = 0.133 nm) was too large to form a ring salt with glucose molecules.23 For the Na+ (rNa+ = 0.103 nm) with the same ionic radius as Ca2+, its electric charge was not high enough to form a relatively stable ring transition state with the glucose molecule.15 This assumption needs to be further confirmed. The effect of the amount of alkali was investigated, as shown in Figure 3. Upon addition of various amounts of Ca(OH)2, the pH values in the reaction solutions remained constant at 11.7 (measured at 288 K) because of the known low solubility of this base in the solution. In a previous report, the retro-aldol

Figure 3. Effect of amount of alkali on the conversion of glucose (Reaction condition: 27 g 30 wt % glucose aqueous solution; CuCr(4), 0.4 g. First step: 413 K, 6 MPa, 2 h. Second step: 493 K, 6 MPa, 5 h, 900 rpm; higher than 99.9% of glucose conversion). 8739



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Table 3. Recyclinga of CuCr(4) Catalysts with or without Ca(OH)2 yield (%) entry

catalysts

1,2-PD

C2−4

C6b

ISO

1,4-SB

oligos.

others

1 2 3 4 5 6

CuCr(4)-R CuCr(4)-RS CuCrCa CuCrCa-SCa CuCrCa-S CuCrCa-SCR

5.4 5.0 52.8 42.3 11.7 11.0

10.1 8.9 76.9 65.1 16.7 14.6

38.9 29.4 20.9 14.5 38.2 61.8

14.3 11.2 − − 5.3 9.5

17.9 12.8 − − 13.3 18.1

30.2 34.6 0.2 10.1 32.4 18.4

9.1 7.8 1.1 2.8 1.3 1.7

a

Reaction conditions: 27 g 30 wt % glucose aqueous solution; CuCr(4), 0.4 g; Ca(OH)2, 0.08 g. First step: 413 K, 6 MPa, 2 h. Second step: 493 K, 6 MPa, 5 h, 900 rpm; higher than 99.9% of glucose conversion; bC6: SOR, MAN, ISO, and 1,4-SB.

condensation was significantly dependent on the concentration of OH− in the solution.14 Nevertheless, the alkalinity of the catalyst also played a major role in the hydrogenolysis, as reported that the NiMg catalyst (Ni/MgO) provided excellent activity for the hydrogenolysis of cellulose.16 The presence of a large amount of solid Ca(OH)2 could also enhance this conversion process. First, the yield of C6 products decreased monotonically by increasing the amount of Ca(OH)2, which was due to the enhancement in the retro-aldol condensation, resulting in an increase in the yield of C2−4 products. More Ca(OH)2 led to the further conversion of C2−4 polyols,24 as indicated by the observed decrease in the yield of C2−4 products and increase in the yield of other chemicals. As in the presence of Ca(OH)2 without Cu−Cr catalyst, severe coking occurred, reflecting that the Cu−Cr catalyst was needed for the conversion of glucose. Meanwhile, the base has a promoting effect on the catalytic performance of Cu−Cr catalyst. 3.3. Recyclability of the Cu−Cr Catalyst with Base. The recyclability of the catalysts was examined, and representative results are shown in Table 3. In the absence of a base, it was not hard to find that the reduced CuCr(4) catalyst (CuCr(4)-R, entry 1) exhibited reasonable recycling ability because only a slight decrease in the total yield after three cycles over the used CuCr(4)-R catalyst (CuCr(4)-RS, entry 2) was observed because of partial deactivation. In the presence of a base (CuCrCa, entry 3), the product distribution and reaction route were changed over the spent CuCrCa catalyst (CuCrCa-S, entry 5) in the recycling. To eliminate the effect of surface deposited carbon species, the calcined and reduced CuCrCa-S catalyst (CuCrCa-SCR, entry 6) was explored for the conversion of glucose under the same conditions. No obvious change in product distribution was observed compared to that of CuCrCa-S catalyst. Taking all this into account, such changes in the product distribution and reaction route for the CuCrCaS catalyst could be assigned to the loss of base because the supernatants (the CuCrCa-S catalyst and calcined CuCrCa-S catalyst separately dispersed into water) were almost neutral. In addition, the ICP result (Cu, 4.4 × 10−3 mg/mL; Cr, 6.2 × 10−4 mg/mL; Ca, 1.6 mg/mL) showed a large amount of Ca leaching, which is explained by the greater solubility of the base in the presence of polyols than in water itself.25 Thus, by adding a certain amount of base into CuCrCa-S catalyst (CuCrCa-SCa, entry 4), the catalytic performance of the CuCrCa-SCa catalyst should be recovered. As expected, the similar product distribution over it was observed compared to that of CuCrCa catalyst. The slight decrease of product yield could be assigned to the change of catalyst chemical−physical properties after reaction, which will be discussed later. To investigate the stability of catalysts, the structure of fresh and used catalysts was examined by XRD and XPS. Figure 4

Figure 4. XRD patterns of CuCr(4) after reduction and reaction with or without Ca(OH)2.

shows the XRD patterns of the Cu−Cr catalysts after reduction and reaction. After reduction, the characteristic diffraction peaks of metal copper together with those of CuO with low intensity were identified. The intensity of these diffraction peaks obviously increased after reaction, indicating a greater degree of crystallinity and crystallite growth. Adding Ca(OH)2 had no obvious effect on the bulk structure of CuCr(4)-R catalyst. No diffraction peaks corresponding to calcium species were observed because of its low content. Similarly, the characteristic peaks of CaO were not presented in the calcined CuCrCa-S catalyst (CuCrCa-SC). To further verify the surface structure of the fresh catalyst and used catalyst, XPS analysis was conducted (Figure 5). After elimination of carbon, the atomic composition is summarized in Table S3 of the Supporting Information. In the reduced catalyst, the atomic content of surface Cu species and the Cu/ Cr ratio were increased because of the distinct reduction properties of CuCr2O4,26 which led to formation of epitaxially bound phases of metallic copper. The atomic content of surface Cu and Cr species was significantly decreased in the used catalysts. The reason is that the used catalysts were covered by oxygenated chemicals, as suggested by C 1s electron spectra, resulting in the increase of surface oxygen content. Significantly, the migration of Cr onto Cu sites occurred during the reaction as reported during the reduction of CuCr2O4·CuO,27 which led to the decrease of the Cu/Cr ratio. One reason for deactivation of the CuCr(4) catalyst is therefore this migration, because single Cr species have no activity in the conversion of glucose as in that of glycerol.18 In the case of CuCr(4) catalyst, the Cu 2p spectra were fitted with two components for the Cu 2p3/2 and satellite peak. The binding energies (BE) of Cu 2p3/2 at 933.2 and 934.7 eV were 8740



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Figure 5. XPS analysis of Cu 2p (a) and Cu L3M45M45 Auger (b) of fresh catalyst and used catalyst.

attributed to Cu2+ species in CuO and CuCr2O4,28,29 respectively, which are characterized by shake up features present at high BE. After reduction and/or reaction, the distinguishable satellite peaks with high BE were also detected, indicating the presence of Cu2+. Slight downward shifts in the BE of Cu2+ were due to the increase of particle size. The peak centered at 932.5 eV corresponded to Cu0 or Cu+ for the CuCr(4)-R, CuCr(4)-RS, and CuCrCa-S catalyst, which need to be verified by the Cu L3M45M45 Auger. As shown in Figure 5b and its corresponding kinetic energy positions in Table S3 of the Supporting Information, large amounts of Cu2+ are present in the CuCr(4)-R catalyst even after high-temperature reduction. However, the content of Cu2+ decreased from 28.7% to 14.7% or 12.3% after reaction, indicating the occurrence of further reduction under high H2 pressure at high reaction temperature. For the CuCr(4)-RS catalyst, the surface Cu2+ species were reduced to Cu+ and Cu0 during the reaction. However, the extent of this reduction was lessened in the presence of a base, resulting in larger amounts of Cu+ in the CuCrCa-S catalyst. The Ca 2p spectra were also recorded for the CuCrCa-S catalyst, as shown in Figure 6. The BE of the two Ca 2p3/2 components were at 347.3 ± 0.2 eV for CaCO3 and 346.7 ± 0.2 eV for Ca(OH)2 species, respectively (the corresponding Ca 2p1/2 peaks were at 3.5 ± 0.2 eV higher BE). The difference in BE between the contributions was in agreement with that

published previously.30 The presence of CaCO3 was due to the carbonation of Ca(OH)2 with trapped CO215 which could be detected in the gas-phase products. After reaction, the reactant, product, or coke might be deposited on the used catalyst, resulting in the deactivation. To further explore these carbon species, each C 1s spectrum of fresh and used catalysts was curve-fitted with three individual components that represent a carbonyl group (CO, peak 1, 288.1 eV), adventitious carbon (C−C, peak 2, 284.5 eV), and carbon present in alcohol or ether groups (C−OH or C−O−C, peak 3, 285.7 eV).31 These spectra and curve fits are shown in Figure 7, and the corresponding compositions are reported in

Figure 7. Carbon 1s electron spectra from fresh catalyst and used catalyst.

Table S4 of the Supporting Information. Spectral contributions from peak 3 (alcohol or ether) in the used catalyst waere significantly greater than those in the fresh catalyst, which can be explained by the deposition of both the reaction products (polyols) and oligomers on the surface of CuCr(4) catalyst. As suggested above, the addition of base has no obvious effect on the stability of Cu−Cr catalyst, although the reaction route is significantly changed in the recycling because of the

Figure 6. Ca 2p electron spectra of the CuCrCa-S catalyst. 8741



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loss of the base. The performance of the spent catalyst could be almost recovered by adding a certain amount of base. However, the partial deactivation of the catalyst is observed, which is mainly attributed to two causes: (1) the migration of Cr onto Cu sites and (2) the coverage of carbon species on active sites. In addition, particle aggregation after reaction could also lead to the decrease of activity in the recycling.

4. CONCLUSIONS Two-step conversion of highly concentrated glucose was performed over Cu−Cr catalyst for the first time, which effectively avoided the formation of coke-like precipitates. The corresponding reaction pathways were also proposed. Adding base significantly changed the reaction route, with the basecatalyzed retro-aldol condensation leading to an increase in the yield of C−C bond cleavage products. Addition of Ca(OH)2 gave the best results compared to those of other bases, indicating that the conversion of glucose depended not only on the hydroxide ion concentration but also on the metal ion of the base. The appropriate metal ionic radius and electric charge may be necessary for the formation of a relatively stable ring transition state with glucose. In the presence of a base, the catalyst exhibited reasonable recyclability, whereas the migration of Cr onto Cu sites and the coverage of carbon species led to the partial deactivation of the catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

Tables listing one-step conversion results of glucose; conversions of glucose in references; surface elemental composition of various Cu oxidation species and carbon oxide species from XPS analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-411-84986353. E-mail: [email protected]. URL: http://finechem.dlut.edu.cn/liangchanghai. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (21073023 and 21373038) and the Fundamental Research Funds for the Central Universities (DUT12YQ03). C.T.W. thanks Dalian University of Technology for a “Sea-sky” professorship.

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REFERENCES

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Two-Step Conversion of Biomass-Derived Glucose with High

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